PDC bits are a type of rotary drag bit used for boring through subterranean rock formations when drilling oil and natural gas wells. As a PDC bit is rotated, typically by rotating a drill string to which it is attached, discrete cutting structures affixed to the face of the bit drag across the bottom of the well, scraping or shearing the formation. PDC bits use cutting structures, referred to as “cutters,” each having a cutting surface or wear surface comprised of a polycrystalline diamond compact (PDC), hence the designation “PDC bit.”
Each cutter of a rotary drag bit is positioned and oriented on a face of the drag bit so that a portion of it, which will be referred to as its wear surface, engages the earth formation as the bit is being rotated. The cutters are spaced apart on an exterior cutting surface or face of the body of a drill bit in a fixed, predetermined pattern. The cutters are typically arrayed along each of several blades, which are raised ridges extending generally radially from the central axis of the bit, toward the periphery of the face, usually in a sweeping manner (as opposed to a straight line). The cutters along each blade present a predetermined cutting profile to the earth formation, shearing the formation as the bit rotates. Drilling fluid pumped down the drill string, into a central passageway formed in the center of the bit, and then out through ports formed in the face of the bit, both cools the cutters and helps to remove and carry cuttings from between the blades.
The shearing action of the cutters on the rotary drag bits is substantially different from the crushing action of a roller cone bit, which is another type of bit frequently used for drilling oil and gas wells. Roller cone bits are comprised of two or three cone-shaped cutters that rotate on an axis at a thirty-five degree angle to the axis of rotation of the drill bit. As the bit is rotated, the cones roll across the bottom of the hole, with the teeth crushing the rock as they pass between the cones and the formation.
PDC cutters are typically made by bonding a layer of PDC, sometimes called a crown or diamond table, to a substrate. PDC, though very hard, tends to be brittle. The substrate, while still very hard, is tougher, thus improving the impact resistance of the cutter. The substrate is typically made long enough to act as a mounting stud, with a portion of it fitting into a pocket or recess formed in the body of the bit. However, the PDC and the substrate structure can be attached to a metal mounting stud. For purposes of the following disclosure, a cutter's “body” refers to any structure that supports the PDC wear surface in the proper position and orientation.
The cutter's PDC wear surface, as mentioned, is comprised of sintered polycrystalline diamond (either natural or synthetic) exhibiting diamond-to-diamond bonding. Polycrystalline cubic boron nitride, wurtzite boron nitride, aggregated diamond nanorods (ADN) or other hard, crystalline materials are substitutes for diamond in at least some applications. A compact is made by mixing the polycrystalline material in powder form with one or more powdered metal catalysts and other materials, forming the mixture into a compact, and then sintering it using high heat and pressure or microwave heating. Sintered compacts of polycrystalline cubic boron nitride, wurtzite boron nitride, ADN and similar materials are, for the purposes of the PDC bit and cutting structures described below, equivalents to polycrystalline diamond compacts and, therefore, references to “PDC” should be construed to refer also to sintered compacts of polycrystalline diamond, cubic boron nitride, wurtzite boron nitride and similar materials unless otherwise indicated. “PDC” will also refer to sintered compacts of these materials with other materials or structure elements that might be used to improve its properties and cutting characteristics. Furthermore, PDC encompasses thermally stable varieties in which a metal catalyst has been partially or entirely removed after sintering.
Substrates for supporting the PDC wear surface or layer are made, at least in part, from cemented metal carbide, with tungsten carbide being the most common. Cemented metal carbide substrates are formed by sintering powdered metal carbide with a metal alloy binder. The composite of the PDC and the substrate can be fabricated in a number of different ways. It may also, for example, include transitional layers in which the metal carbide and diamond are mixed with other elements for improving bonding and reducing stress between the PDC and substrate.
Each PDC cutter is fabricated as a discrete piece, separate from the drill bit. Because of the processes used for fabricating them, the PDC layer and substrate typically have a cylindrical shape, with a relatively thin disk of PDC bonded to a taller or longer cylinder of substrate material. The resulting composite can be machined or milled to change its shape. However, the PDC layer and substrate are typically used in the cylindrical form in which they are made.
When the body of a cutter is affixed to the face of the drill bit, the body of the cutter occupies a recess or pocket formed in the cutting face. In some types of bits, a separate pocket or recess is formed for each cutter when the body is fabricated, and the body of the PDC cutters is then press fitted or brazed in the recess to hold it in place. However, in the case of matrix body drill bits, which are made by filling a graphite mold with hard particulate matter such as powdered tungsten, and infiltrating the particulate matter with a metal alloy that forms a matrix in which the particulate matter is suspended, the cutters could be placed in the mold before infiltration.